This invention relates to a gas discharge device for causing a gas discharge to occur in a hollow space filled with a gas. It should be noted here that the concept of the term "gas discharge device" covers a sputtering device, a plasma chemical vapor deposition device, a dry etching device, or the like, although the sputtering device will be exemplified throughout the instant specification.
A conventional sputtering device of the type described is for use in depositing a layer on a substrate by sputtering a material of a target onto the substrate. For this purpose, the sputtering device comprises a chamber defining a hollow space, an anode electrode in the hollow space, and a cathode electrode opposite to the anode electrode in the hollow space, and is accompanied by a main power source for providing a voltage between the anode and the cathode electrodes. The substrate is placed on the anode electrode while the cathode electrode is operable as the target.
In order to carry out sputtering operation, a gas discharge should be caused to occur in the hollow space between the anode and the cathode electrodes. In order to deposit an excellent layer on the substrate, the gas discharge must be stable during the sputtering operation. However, the gas discharge is subjected to influence of various factors, such as a geometrical configuration of the anode and the cathode electrodes, a pressure in the hollow space, a voltage applied between the anode and the cathode electrodes, a distance therebetween, and the like. Moreover, an initial voltage for causing the gas discharge to start is different from a voltage for causing it to continue. It is therefore difficult to keep the gas discharge stable.
It is mentioned here that the Paschen's law is well known in the art to define a relationship among the initial voltage, the distance, and the pressure. According to the Paschen's law, the initial voltage for the gas discharge becomes low with a rise in the pressure and with an increase in the distance.
In accordance with the teaching of Paschen's law, the distance between the anode and the cathode electrodes may be widened once from a predetermined distance at a start of the gas discharge and be returned to the predetermined distance after the start of the gas discharge. Provision should, however, be made of a mechanism for moving one of the electrodes nearer to the other and further therefrom on controlling the distance. The mechanism is intricate in structure. The control of the distance is difficult and time consuming.
Alternatively, an auxiliary electrode is placed to cause the gas discharge to start. An extra power source should be prepared in addition to the main power source because a high voltage is impressed on the auxiliary electrode. A restriction is inevitably imposed on design of the target because the auxiliary electrode must be adjacent to the target or the cathode electrode.
As will later be described with reference to one of nearly ten figures of the accompanying drawing, it is possible to change the gas pressure in the hollow space at the start of the gas discharge. More specifically, the pressure is increased to a high pressure or an initial pressure higher than a normal pressure and is returned to the normal pressure after the start of the gas discharge. It is, however, difficult to automatically increase the pressure to the initial pressure at the start of the gas discharge because the initial pressure is fairly higher than the normal pressure.